Coated powder for electrolyte matrix for carbonate fuel cell

ABSTRACT

A plurality of electrolyte carbonate-coated ceramic particle which does not differ significantly in size from that of the ceramic particle and wherein no significant portion of the ceramic particle is exposed is fabricated into a porous tape comprised of said coated-ceramic particles bonded together by the coating for use in a molten carbonate fuel cell.

The present invention is directed to a coated powder for preparing anelectrolyte porous matrix, i.e. electrolyte porous tape, for a carbonatefuel cell.

The anode and cathode of state-of-the-art carbonate fuel cells areseparated by an electrolyte layer, commonly called a "tile". The tile istypically composed of LiAlO₂ or SrTiO₃ particles which comprise a porousceramic matrix, the interstices of which are filled with a carbonateelectrolyte mixture.

One of the principal problems associated with the tile is fabrication bya commercially attractive method, consistent with desirablecharacteristics, i.e., porosity, pore size distribution, and strength.

Recent work in this area has been directed towards casting of the matrixceramic powder by vacuum casting, tape casting, and electrophoreticdeposition. Although all three methods are commercially viableprocesses, all three methods produce undesirable products. The pivotalproblem of the structures produced by these processes is that theyfracture when the electrolyte is introduced (referred to in the art asimpregnating).

In accordance with the present invention, a porous electrolyte matrix,i.e. tape, is formed which overcomes this problem of cracking uponelectrolyte carbonate impregnation in the fuel cell. In this invention,the ceramic particles are first coated with a layer of carbonate.

Briefly stated, the present invention is directed to the production ofelectrolyte carbonate-coated ceramic particles which are fabricated intoa porous electrolyte tape for use as the porous matrix of the tile in amolten carbonate fuel cell.

In the accompanying FIGURE, peak cell voltage is plotted for a number ofindividual operating molten carbonate fuel cells, each point in theFIGURE indicates a molten carbonate fuel cell. The numbered points inthe FIGURE illustrate the present invention.

One embodiment of the present invention comprises a process forproducing coated ceramic particles for fabrication into a porouselectrolyte tape for a molten carbonate fuel cell which comprisesforming a mixture of electrolyte carbonate and ceramic particles whereinthe carbonate ranges from about 5% by volume to about 30% by volume ofthe total amount of said mixture, said ceramic particle ranging in sizefrom about 0.1 micron to about 5 microns, said ceramic particles notbeing significantly deleteriously effected by said molten carbonate fuelcell, heating said mixture to a temperature at which the carbonate ismolten but at which it does not vaporize significantly, coating saidceramic particle with said molten carbonate leaving no significantportion of said ceramic particle exposed, and allowing the resultingcarbonate-coated particles to cool to solidify the carbonate.

Another embodiment of the present invention comprises coated particlesfor fabrication into a porous electrolyte tape composed of said coatedparticles bonded together by said coating, said coated particles beingcomprised of a plurality of electrolyte carbonate-coated ceramicparticle which does not differ significantly in size from said ceramicparticle and wherein no significant portion of said ceramic particle isexposed, said electrolyte carbonate ranging from about 5% by volume toabout 30% by volume of the total amount of said coated particles, saidceramic particles not being significantly deleteriously effected by saidmolten carbonate fuel cell, said ceramic particle ranging in size fromabout 0.1 micron to about 5 microns.

In carrying out the present process, the particular electrolytecarbonate or carbonate mixture used depends largely on the fuel cellbeing constructed. Generally, it is selected from the group consistingof lithium carbonate, sodium carbonate, potassium carbonate, mixturesthereof, and mixtures thereof with strontium carbonate. The presentelectrolyte carbonate is a solid at room temperature and its meltingpoint depends on its particular carbonate composition. All of thepresent electrolyte carbonates are molten at the operating temperatureof the fuel cell which usually ranges from about 500° C. to about 700°C. In most instances, and preferably, the present carbonate is a mixtureof about 62 mole % lithium carbonate and about 38 mole % potassiumcarbonate, which has a melting point of about 500° C.

The present ceramic particles are not significantly deleteriouslyeffected by the molten carbonate fuel cell, i.e. by the operatingenvironment of the molten carbonate fuel cell. Specifically, the presentceramic particles are stable to the operating environment of the moltencarbonate fuel cell. Representative of the present ceramic particles arethose selected from the group consisting of lithium aluminate, strontiumtitanate and mixtures thereof.

The size or size distribution of the ceramic particles depends largelyon the porosity desired in the porous electrolyte tape fabricatedtherefrom. Generally, the ceramic particles range in size from about 0.1micron to about 5 microns, and preferably from about 0.2 microns toabout 2 microns. Preferably, ceramic particles of distributed size areused, such as for example, 25% by volume of ceramic particles of a sizeof about 0.2 micron and 75% by volume of particles of a size of about1-2 microns to produce a porous tape of distributed pore size.

The ceramic particles are coated with the electrolyte carbonate toproduce carbonate coated-particles wherein the ceramic particle istotally coated by carbonate, or wherein no significant portion of theceramic particle is exposed. The size of the carbonate-coated ceramicparticle is essentially the same as that of the ceramic particle, i.e.,it does not differ significantly in size from that of the ceramicparticle.

The coated ceramic particles are produced by coating the ceramicparticles with the electrolyte carbonate in molten form and allowing theresulting material to cool and solidify. Preferably, a particulatemixture of the electrolyte carbonate and ceramic particles is formed,and although the size of the carbonate particles is not critical, it ispreferably less than about 50μ. The mixture is heated at ambientpressure in air to a temperature at which the carbonate is molten, butat which it does not vaporize significantly. Preferably, the mixture isheated to a temperature ranging from about 500° C. to about 650° C.There is no significant advantage in using temperatures higher thanabout 650° C. The mixture is maintained at the temperature at which thecarbonate is molten for a time period sufficient for the molten or fluidcarbonate to coat the particles completely or to leave no significantportion thereof exposed. The resulting mass, i.e. carbonate-coatedparticles, is allowed to cool, preferably to room temperature, tosolidify the carbonate, and then it is lightly comminuted producing freeflowing coated particles wherein no significant portion of the ceramicparticle is exposed.

The present coated ceramic particles can be fabricated into a porouselectrolyte tape for use in a carbonate fuel cell by a number oftechniques. Specifically, the present porous electrolyte tape iscomprised of a plurality of electrolyte carbonate-coated ceramicparticle bonded together by the electrolyte carbonate coating and havingno significant portion of the ceramic particle exposed, said ceramicparticles not being significantly deleteriously effected by said moltencarbonate fuel cell and ranging in size (e.g. diameter or lineardimension) from about 0.1μ to about 5μ, said carbonate ranging fromabout 5% by volume to about 30% by volume of the total volume of saidcoated ceramic particles, and said coated ceramic particle not differingsignificantly in size from said ceramic particle (e.g. a 30% volumeincrease generally will correspond to less than a 10% diameterincrease).

The present electrolyte tape has a thickness and porosity which dependlargely on the requirements of the molten carbonate fuel cell.Specifically, the present porous electrolyte tape has a uniform orsubstantially uniform thickness of less than about 30 mils (760microns), i.e., it has a thickness which does not differ significantly.Generally, its thickness ranges from about 7 mils (180 microns) to about20 mils (500 microns). Generally, it is self-supporting. It has aporosity ranging from about 30% by volume to about 65% by volume, andpreferably about 50% by volume, of the total volume of the tape.Generally, it has a pore size ranging from about 0.2 micron to about 2microns, and a median pore size of about 1 micron. It is useful as anelectrolyte supporting matrix in a molten carbonate fuel cell.

One embodiment for producing the present porous tape comprises admixingthe coated ceramic particles with an organic binding agent, shaping themixture into a self-supporting tape, and heating the tape to decomposeand vaporize away the binding agent producing the present porouselectrolyte tape. The binding agent containing tape is flexible and ofuniform or substantially uniform thickness of less than about 30 mils,i.e. it has a thickness which does not differ significantly. The presentporous tape has a thickness not significantly different from that of thebinding agent-containing tape.

The binding agent is used in an amount sufficient to bind the coatedparticles together to produce a self-supporting tape from which it canbe heat-decomposed and vaporized away leaving no significant residueproducing the present porous tape. Generally, the binding agent is usedin an amount ranging from about 40% by volume to about 65% by volume,and preferably about 50% by volume, of the total volume of binding agentand carbonate-coated ceramic particles.

The binding agent, i.e. binding agent composition, is a solid at roomtemperature, melts at an elevated temperature below 400° C., anddecomposes below about 400° C. vaporizing away leaving no significantresidue thereof. Representative of the present organic binding agents ispolyethylene having a melting point of about 137° C., polypropylene witha melting point of about 176° C. and polybutylene with a melting pointof about 44° C.

Depending largely on the shaping technique and binding agentcomposition, an organic plasticizer may be used as part of the bindingagent composition to aid in the production of the bindingagent-containing tape. Such plasticizer should be fluid at an elevatedtemperature and heat decomposable at an elevated temperature below 400°C. vaporizing away leaving no significant residue. Generally, theplasticizer ranges up to about 18% by weight of the binding agent.Representative of the plasticizers is paraffin and dioctyl phthalate.

The binding agent composition should have no significant deleteriouseffect on the coated particles.

Shaping of the mixture of binding agent and coated particles to producea binding agent-containing tape can be carried out by a number oftechniques. In one shaping embodiment, the mixture of coated particlesand binding agent is heat shaped by passing it through hot stainlesssteel rollers heated to a temperature at which the binding agent is hotand pliable. The thickness of the tape is controlled by setting thedistance between rollers.

In another shaping embodiment, the binding agent and coated particlesare admixed with an organic liquid medium which has no significantdeleterious effect on the coated particles and which is a suspendingmedium for the binding agent, such as, for example, toluene forpolyethylene. The resulting suspension of binding agent and suspendedcoated particles is tape cast, usually by doctor blading the suspensionon a substrate in the thickness desired of the present porous tape, andallowing the coating to dry leaving a flexible tape which can be peeledaway from the substrate.

The binding agent-containing tape is heated to decompose and vaporizeaway the binding agent leaving no significant residue thereof to producethe present porous tape. Preferably, before the carbonate fuel cell isplaced in operation, the binding agent-containing tape is placed in thecell and heated therein to decompose and vaporize away the binding agentproducing the present porous tape.

Another embodiment of the present invention comprises forming the poroustape by vacuum casting. Preferably, in this technique, the carbonatecoated particles are suspended in an organic liquid which has nosignificant deleterious effect thereon, and the suspension is vacuumcast, i.e. vacuum filtered, as a substantially uniform layer in athickness desired of the present porous tape, on a flat piece of filterpaper on which it is dried. The filter paper is peeled away, leaving thepresent porous tape. Most preferably, the suspension is vacuum cast ontoa face of one of the electrodes to be used in the fuel cell, and in suchinstance the deposited layer should be coextensive with the face of theelectrode. The deposited layer is dried producing a composite of thepresent porous tape and the electrode which can be placed directly intothe fuel cell.

The invention is further illustrated by the following tabulatedexamples:

In Table I, a number of batches of electrolyte carbonate coated ceramicparticles given as Lot No. were prepared.

The procedure used to produce each given Lot No. was the same except asindicated in Table I.

The ceramic particles in Table I were lithium aluminate of the givenmedian particle size, and in Table I, the batch size in grams is that ofthe LiAlO₂ and carbonate, i.e. carbonate-coated LiAlO₂ particles.

The carbonate in Table I was comprised of 62 mole % lithium carbonateand 38 mole % potassium carbonate and is based on the total volume ofthe given LiAlO₂ batch size and carbonate.

To produce each given Lot No. of Table I, the carbonate and the lithiumaluminate particles were dry ball milled using alumina grinding mediaand using an equal part by weight of grinding media to the powder beingmilled. Milling was carried out at room temperature for 3 hours.

The resulting mixture was placed in an alumina crucible and an aluminalid was placed thereon. The mixture was heated in air at ambientpressure to the given temperature at which the carbonate was molten andat which it was maintained for the given time period. Within the giventime period, the molten carbonate coated the lithium aluminate particlestotally or did not leave any significant portion thereof exposed. At theend of the given time period, the resulting mass was allowed to cool toroom temperature to solidify the carbonate.

The solidified mass was lightly comminuted by placing it on an 80 meshnylon screen and lightly brushing it through the screen with a rubberspatula. The resulting free flowing powder of each Lot No. of Table Iwas comprised of a plurality of carbonate-coated lithium aluminateparticle wherein no significant portion of the lithium aluminateparticle was exposed. The coated particles of each Lot No. of Table Idid not differ significantly in size from that of the lithium aluminateparticles. All of the examples, i.e. Lot Nos. of Table I, illustrate thepresent invention.

                  TABLE I                                                         ______________________________________                                                       Carbonate                                                                     Coated                                                              LiAlO.sub.2                                                                             LiAlO.sub.2                                                         Median    Batch                                                          Lot  Particle  Size      Carbonate                                                                             Temperature                                                                            Time                                No.  Size (μ)                                                                             (grams)   (vol. %)                                                                              (°C.)                                                                           (hrs)                               ______________________________________                                        C1   1         100       10      550      4                                   C2   0.2       100       10      550      4                                   C3   1         100       10      550      6                                   C4   2.1       213       10      650      4                                   C5   0.2       100       10      650      4                                   ______________________________________                                    

Each run of Table II illustrates the production of the present porouselectrolyte tape and the cell in which it was used as the electrolytesupporting matrix of the tile.

Specifically in Table II, the coated ceramic particles prepared in TableI were used and are identified by Lot No. Also, in Table II, thepolyethylene had a melting point of 137° C. and the polybutylene had amelting point of 44° C. The paraffin had a melting point of 50° to 57°C. and was used in the given amount based on the weight of thepolybutylene. All of the binding agent compositions in Table II wereheat decomposable below 400° C. leaving no significant residue.

The procedure used in each tabulated run of Table II was the same unlessotherwise indicated. A conventional hot roll mill with stainless steelrollers preheated to 230° F. was used with the rollers initially set toalmost touch, i.e. about 2-4 mils apart. The binding agent compositionwas particulate in form and was dry ball-milled with the given Lot No.of coated ceramic particles in the given amount to produce asubstantially uniform mixture. The mixture was poured on the hot rollswhere it began to melt and the rollers were then turned on. The mixturewas rolled on the hot rolls several times, cut off with a cutting blade,rolled into a ball and again passed through the hot rolls several timesto get a uniform mixture. The rolls were then set a distance apartequivalent to the tape thickness wanted and the front roller was set ata rate faster than the back one so that the tape would adhere to thefront roller. The rollers, preheated to 230° F., were then turned on andthe tape formed on the front roller. The rollers were then furtherseparated, turned off, the tape cooled to room temperature and peeledoff the front roller. In each run, the resulting bindingagent-containing tape was flexible, rubbery, and had a thickness whichwas uniform or which did not differ significantly. In each run, theresulting binding agent-containing tape was about 17" long and about 7"wide.

A portion of each resulting binding agent-containing tape was weighed,heated in air from 25° C. to 400° C. in 13 hours, from 400° C.-650° C.in 6 hours, held at 650° C. for 2 hours and then furnace-cooled to roomtemperature. Each resulting tape was weighed and the loss in weightcaused by the heating, i.e. heat decomposition and vaporization away ofthe binding agent composition, was equivalent to the theoretical amountof binding agent that had been present in the tape before the heatingindicating that no residue or no significant residue of binding agentremained and that the heating had no significant effect on thecarbonate-coated particles. Specifically, each resulting tape wascomprised of carbonate-coated LiAlO₂ particles bound together by thecarbonate coating and showed no significant exposure of the LiAlO₂particles. Each resulting tape was porous, self-supporting, and itsthickness is given in Table II. Each resulting porous tape was ofuniform thickness, or had a thickness which did not differsignificantly, and also, its thickness did not differ significantly fromthat of the binding agent-containing tape.

The pore size distribution of each resulting tape was determined bymercury intrusion porosimetry from which its median pore size andporosity were calculated. The results are given in Table II. Theporosity in Table II is % by volume of the total volume of the poroustape.

                                      TABLE II                                    __________________________________________________________________________                                   Median                                                             Thickness                                                                           Porosity                                                                           Pore Size                                                                          Cell                                      Run                                                                              Composition      (mils)                                                                              (%)  (μ)                                                                             Tests                                     __________________________________________________________________________    1  50 vol. %                                                                           polyethylene                                                                             9     49   1.1  #43                                          50 vol. %                                                                           Lot C1                                                               2  50 vol. %                                                                           polyethylene                                                                             9     49   1.1  #45                                          50 vol. %                                                                           Lot C1                                                               3  50 vol. %                                                                           polyethylene                                                                    75 vol. % Lot C1                                                   50 vol. %           19    44   0.55 #49                                                  25 vol. % Lot C2         #55                                       4  50 vol. %                                                                           polybutylene                                                                    75 vol. % Lot C1                                                   50 vol. %           19    44   0.53 #52                                                  25 vol. % Lot C2         #54                                                                           #56                                       5  50 vol. %                                                                           polybutylene & 12                                                       wt. % paraffin                                                                        75 vol. % Lot C4                                                   50 vol. %           15    51   0.87 not cell                                             25 vol. % Lot C5         tested yet                                6  50 vol. %                                                                           polyethylene                                                                    75 vol. % Lot C3                                                   50 vol. %           23    47   0.87 #62                                                  25 vol. % Lot C5         #63                                       7  50 vol. %                                                                           polybutylene & 12                                                       wt. % paraffin                                                                        75 vol. % Lot C4                                                   50 vol. %           19    --   --   #53                                                  25 vol. % Lot C5         #57                                       8  50 vol. %                                                                           polybutylene & 18                                                       wt. % paraffin                                                                        75 vol. % Lot C4                                                   50 vol. %           20    45   0.94 #58                                                  25 vol. % Lot C5         #59                                                                           #60                                       __________________________________________________________________________

Also, in each tabulated run of Table II, before start up of the cell, asquare of the binding agent-containing tape, 43/4"×43/4", was placed ineach cell. Enough carbonate composed of 62 mole % lithium carbonate and38 mole % potassium carbonate was then added to fill the porosity of theporous tape in each cell. Each cell was then heated from 25° C. to 400°C. at a rate of 15.4° C./hr, from 400° C. to 500° C. at a rate of 30°C./hr and from 500° C. to 650° C. at a rate of 90° C./hr. From previoustests, it had been determined that at 400° C. all of the binding agenthad decomposed and vaporized away producing the present porous tape. Atabout 496° C., the carbonate began to melt and at about 510° C.-520° C.the molten carbonate had filled the porosity of each tape. At 650° C.,each cell was placed on load for testing.

The accompanying FIGURE shows the performance of the cells of Table IIalong with a number of other molten carbonate fuel cells. Specifically,each point on the FIGURE illustrates the performance of a moltencarbonate fuel cell. The numbered points on the FIGURE correspond to thecell numbers of Table II. The points in the FIGURE without a number,i.e. state of the art cells, were cells which did not differsignificantly from the numbered cells except that they did not containthe present porous tape as an electrolyte supporting matrix.

The performance of all of the cells of the FIGURE were tested insubstantially the same manner. Specifically, each cell had a medium BTUfuel wherein the anode gas was comprised of 48% by volume H₂, 32% byvolume CO₂ and 20% by volume H₂ O, and the cathode gas was comprised of20% by volume CO₂, 12% by volume O₂ and 20% by volume H₂ O balance N₂.The fuel and oxidant had a flow rate such that at load it would use 75%of the H₂ and 50% of the O₂, respectively.

The FIGURE shows individual cell peak performance at 650° C. at a loadof 160 ma/cm². The FIGURE shows that for those cells that did notutilize the present porous tape, there is widely ranging cellperformance most of which was below the goal for the cell of 0.725 volt.In contrast, for the numbered cells of the FIGURE, i.e. those containingthe present porous tape, the performance was much more consistent and inall cases equivalent to or greater than the goal of 0.725 volts.

What is claimed is:
 1. A porous electrolyte tape structure forsupporting electrolyte in a molten carbonate fuel cell consistingessentially of a plurality of electrolyte carbonate-coated ceramicparticles having no significant portion of the ceramic particlesexposed, said ceramic particles not being significantly deleteriouslyeffected by said molten carbonate fuel cell, said ceramic particlesranging in diameter from about 0.1 micron to about 5 microns, saidelectrolyte carbonate coating consisting essentially of a carbonateuseful as an electrolyte in a molten carbonate fuel cell and rangingfrom about 5% by volume to about 30% by volume of said coated ceramicparticle, said coated ceramic particle not differing significantly indiameter from said ceramic particle, said tape structure having athickness substantially greater than the diameter of the ceramicparticles and said thickness encompassing a plurality of said particles,said porous tape having a substantially uniform thickness of less than30 mils and a porosity ranging from about 30% by volume to about 65% byvolume of the total volume of said tape.
 2. A porous electrolyte tapefor supporting electrolyte in a molten carbonate fuel cell consistingessentially of a plurality of electrolyte carbonate-coated ceramicparticles and having no significant portion of the ceramic particleexposed, said ceramic particles being selected from the group consistingof lithium aluminate, strontium titanate and mixtures thereof andranging in size from about 0.2 micron to about 2 microns, saidelectrolyte carbonate coating consisting essentially of a carbonateuseful as an electrolyte in a molten carbonate fuel cell and rangingfrom about 5% by volume to about 30% by volume of said coated ceramicparticles, said coated ceramic particle not differing significantly insize from said ceramic particle, said porous tape having a substantiallyuniform thickness of less than 30 mils and a porosity ranging from about30% by volume to about 65% by volume of the total volume of said tape.3. A composite useful in a molten carbonate fuel cell comprising of anelectrode carrying a porous electrolyte tape on one face thereof, saidporous tape being coextensive with said face of said electrode, saidporous tape having a substantially uniform thickness of less than 30mils and a porosity ranging from about 30% by volume to about 65% byvolume of the total volume of said tape, said porous tape consistingessentially of electrolyte carbonate-coated ceramic particles wherein nosignificant portion of said ceramic particle is exposed, saidelectrolyte carbonate coating consisting essentially of a carbonateuseful as an electrolyte in a molten carbonate fuel cell, said ceramicparticles ranging in size from about 0.1 micron to about 5 microns, saidceramic particles not being significantly deleteriously effected by saidmolten carbonate fuel cell, said electrode being useful as an electrodein a molten carbonate fuel cell.
 4. The composite according to claim 3wherein said ceramic particles are selected from the group consisting oflithium aluminate, strontium titanate and mixtures thereof.
 5. A tapefor use in a molten carbonate fuel cell consisting essentially of aplurality of electrolyte carbonate-coated ceramic particles incombination with an organic binding agent decomposable below 400° C. toleave a porous tape structure for supporting electrolyte, said amount ofbinding agent ranging from about 40% to about 65% of the total volume ofbinding agent and carbonate-coated ceramic particles, said ceramicparticles having no significant portion thereof exposed and not beingsignificantly deleteriously effected by said molten carbonate fuel cell,said ceramic particles ranging in diameter from about 0.1 micron toabout 5 microns, said electrolyte carbonate being selected from thegroup consisting of lithium carbonate, sodium carbonate, potassiumcarbonate, mixtures thereof, and mixtures thereof with strontiumcarbonate, said electrolyte carbonate ranging from about 5% by volume toabout 30% by volume of said coated ceramic particles, said coatedceramic particle not differing significantly in diameter from saidceramic particle, said tape structure having a thickness substantiallygreater than the diameter of the ceramic particles and said thicknessencompassing a plurality of said particles, said porous tape having asubstantially uniform thickness of less than 30 mils and a porosityranging from about 30% by volume to about 65% by volume of the totalvolume of said tape.
 6. A composite useful in a molten carbonate fuelcell comprising an electrode carrying a porous electrolyte tape on oneface thereof, said porous tape being coextensive with said face of saidelectrode, said porous tape having a substantially uniform thickness ofless than 760 microns and a porosity ranging from about 30% by volume toabout 65% by volume of the total volume of said tape, said porous tapeconsisting essentially of electrolyte carbonate coated-ceramic particleswherein no significant portion of said ceramic particle is exposed, saidelectrolyte carbonate being selected from the group consisting oflithium carbonate, sodium carbonate, potassium carbonate, mixturesthereof, and mixtures thereof with strontium carbonate, said ceramicparticles ranging in diameter from about 0.1 microns to about 5 microns,said ceramic particles not being significantly deleteriously effected bysaid molten carbonate fuel cell, said tape structure having a thicknesssubstantially greater than the diameter of the ceramic particles andsaid thickness encompassing a plurality of said particles, saidelectrode being useful as an electrode in a molten carbonate fuel cell.